Wet processing of semiconductor substrates, such as wafers, flat panels, and other electronic component precursors is used extensively during the manufacture of, for example, integrated circuits. Preferably, wet processing is carried out to prepare the semiconductor substrates for processing steps such as diffusion, ion implantation, epitaxial growth, chemical vapor deposition, and hemispherical silicon grain growth, or combinations thereof. During wet processing, the semiconductor substrates are contacted with a series of process solutions. The process solutions may be used, for example, to etch, to remove photoresist, to clean, or to rinse the semiconductor substrates. See, e.g., U.S. Pat. Nos. 4,577,650; 4,740,249; 4,738,272; 4,856,544; 4,633,893; 4,778,532; 4,917,123; and EPO 0 233 184, assigned to a common assignee, and Burkman et al., Wet Chemical Processes-Aqueous Cleaning Processes, pg 111-151 in Handbook of Semiconductor Wafer Cleaning Technology (edited by Werner Kern, Published by Noyes Publication Parkridge, New Jersey 1993), the disclosures of which are herein incorporated by reference in their entirety.
There are various types of systems available for wet processing. For example, the semiconductor substrates may be processed in a single vessel system closed to the environment (such as a Full-Flow.TM. system supplied by CFMT Technologies), a single vessel system open to the environment, or a multiple open bath system (e.g., wet bench) having a plurality of baths open to the atmosphere.
Following processing, the semiconductor substrates are typically dried. Drying of the semiconductor substrates can be done using various methods, with the goal being to ensure that there is no contamination created during the drying process. Methods of drying include evaporation, centrifugal force in a spin-rinser-dryer, steam or chemical drying of wafers, including the method and apparatus disclosed in, for example, U.S. Pat. No. 4,778,532.
A common problem encountered in the wet processing of semiconductor substrates is obtaining repeatable processing results (i.e. process control) for all surfaces of a single semiconductor substrate, between semiconductor substrates in a single batch, and between batches of semiconductor substrates that are processed in the same manner. For example, when semiconductor wafers are etched to remove oxides, it is desirable that the thickness of etching is substantially the same on all surfaces of a single wafer, as well as between wafers within the same batch. Additionally, it is desired that wafers in different batches being processed under the same etching conditions do not vary substantially in etching thickness (i.e., batch to batch variation).
Traditionally, process control in semiconductor wet processing is completed through the use of "monitor semiconductor substrates." Monitor semiconductor substrates are processed in the equipment using the same manufacturing conditions as the production semiconductor substrates. The monitor semiconductor substrates are then tested to ensure that the manufacturing process is running within its specified limits. However, the use of monitor semiconductor substrates can be costly. For example, the use of monitor semiconductor substrates leads to lost production time as well as wasted raw materials in processing the monitor semiconductor substrates.
One way to eliminate or reduce the use of monitor semiconductor substrates is to monitor processing conditions in the processing vessel and make adjustments to the processing conditions during processing. For example in an etching process for semiconductor substrates, it is known that etching thickness is a function of etching time, temperature, and chemical concentration of the etching agent. Also for example, in cleaning processes, such variables as cleaning time, temperature, use of megasonic energy, and chemical concentration can have an impact on the uniformity and efficiency of cleaning of the semiconductor substrates. Thus, processing results can be controlled through such parameters as temperature, chemical concentration, and processing time. While solution temperature and processing time can readily be controlled in most wet processing systems, the measuring and controlling of chemical concentrations has been problematic. Thus, much effort has focused on developing systems and methods for determining chemical concentrations for improved process control in wet processing systems.
For example, in U.S. Pat. No. 5,472,516 to Hanson et al., ("Hanson") a control strategy is proposed for measuring and maintaining the concentration of chemicals in a bath. In Hanson, the concentrations of ammonium hydroxide and hydrogen peroxide in an SC1 cleaning solution were monitored by measuring the pH and conductivity of the SC1 cleaning. The conductivity was used to control the addition of ammonia to the bath, and the pH was used to control the addition of hydrogen peroxide to the bath. The life of the SC1 solution was extended by the process. IR spectrometric monitors have also been used to monitor chemical concentrations in open bath systems.
In open bath systems such as the system used by Hansen, monitors may be readily placed within the bath in-situ, providing the user with real-time chemical concentration information. However, even in an open bath system, in-situ monitors, may not accurately measure chemical concentrations. For example, when there is more than one chemical present, one may not be able to accurately measure the concentration of a chemical (such as a weak acid or base) due to the presence or interaction of other chemicals (such as a strong acid or strong base) present in the bath. Often, more than one monitor may be needed to measure the concentrations of different chemicals, leading to increased costs for purchasing and maintaining the monitors. Equipment for measuring concentrations directly, such as conductivity meters, can also be unreliable. Thus there is a need for simplified ways to determine chemical concentrations in wet processing systems.
Determining chemical concentrations in a single pass wet processing vessel (where a solution is passed once through the vessel) can be further problematic. For example, in many single pass wet processing vessels, the placement of a concentration measuring device in the vessel will disrupt the flow pattern of process solution resulting in nonuniform contacting of the process solution with the semiconductor substrates. A solution would be to place the measuring devices upstream or downstream of the process vessel. However, when the process solution contains mixtures of chemicals, more than one measuring device will most likely be needed, leading to increased costs for purchasing and maintaining several measuring devices. Additionally, it may not even be possible to accurately measure the concentration of each chemical in the process solution due interactions between the chemicals or equipment reliability problems.
Thus, there is a need for simpler methods and systems for determining chemical concentrations of wet processing streams used in a wet processing system. Further, there is a need for simpler methods and systems for controlling the processing of semiconductors in a wet processing system.